Walls made from gypsum wall board or drywall are conventionally constructed by affixing the boards to studs or joints and filling and coating the joints with a specially prepared adhesive called “joint compound.” This process is also used to make repairs of defects, such as holes and dents, including those around electrical boxes, piping and duct work, as well as corners created by the intersection of drywall boards.
Drywall tape adds strength and crack resistance as well as smooth concealment at flat joints and inside corners. Conventionally, two types of drywall tape have been employed—a simple kraft paper strip which is adhered to the drywall surfaces by a bedding coat of joint compound or “mud,” and glass fiber tape, which can be applied with joint compound or self-adhered. Kraft paper tape must be carefully positioned and care must be taken not to discharge the mud onto non-working surfaces. In addition, once the paper drywall tape has been applied, one must wait as much as a day for the compound to dry before a final surface coat of compound can be applied. Glass fiber tape, on the other hand, provides exceptional wet and dry strength and resists stretching and wrinkles. It can be laid flat and resists tearing under load.
A joint treatment system which includes reinforcing tape and joint compound, must provide joints as strong as the gypsum board itself. Otherwise, normal structural movement in the wall or ceiling assembly can result in the development of cracks over the finished joint.
It has been discovered that certain types of fiberglass leno-weave mesh tape when used with conventional joint compounds are more prone to cracking than joints finished with paper tape and conventional joint compounds. Because of this, some manufacturers, such as United States Gypsum Company, Chicago, Ill., have manufactured glass fiber tapes with cross-fiber construction to provide greater drywall joint strength than conventional fiberglass leno-weave mesh tapes. Such tapes include Sheetrock® brand fiberglass drywall tape and Imperial® brand tape. The Imperial® brand tape includes an open weave of glass fibers (100 meshes per square inch) which is coated with a binder and slit to roll width. Spirally woven (leno) long strands and the binder coating reduce edge raveling and fraying and keep the loose threads from defacing finishing surfaces.
Still another drywall tape that has been commercially accepted is Fibatape® glass tape available through San Gobain Technical Fabrics, Ontario, Canada. This product is an SBR rubber-coated glass fiber tape with a self-adhesive backing. The tape is self-adhered to a drywall seam, and then covered with layers of drywall compound. Samples of a drywall joint made with Fibatape® tape have been tensile tested in accordance with ASTM C 474 (Appendix) which measures the strength to first crack of a tape-compound sample coated in electrically conductive paint. The strength is measured until the first crack in the paint occurs, which breaks the electrical continuity along the surface and registers the ultimate tensile load. Failure in Fibatape® joint tape tensile specimens is observed at the SBR rubber film where the joint compound separates from the film at failure. This suggests that typical taped joints in glass tape-drywall systems do not optimize the strength of the glass-joint compound composite, since tensile loads tend to separate the glass fibers from the joint compound matrix instead of transferring these loads to the glass fibers themselves.
Accordingly, there is a need for creating a higher strength wall board joint composite system employing glass fiber tape. Such a system should be chemically compatible with ready mixed or powder joint compound compositions and should not substantially detract from the already established benefits of using glass fiber tape over kraft paper tape.